"The Dark Tower was described as existing on a massive scale so large it was almost surreal, although Tolkien does not provide much detail beyond its size and immense strength. Since it had a "topmost tower" it presumably had multiple towers. It is otherwise described as dark and surrounded in shadow, so that it could not be clearly seen.
"..rising black, blacker and darker than the vast shades amid which it stood, the cruel pinnacles and iron crown of the topmost tower of Barad-dûr..." " - Wikipedia.